1. Field of the Invention
The present invention relates to a toner for developing electrostatic images in electrophotography, electrostatic recording or electrostatic printing, to a developer which comprises the toner, and to an image-forming process and image-forming apparatus using the developer which comprises the toner. More specifically, the present invention relates to a toner for developing an electrostatic image used in copiers, laser printers and fax machines that utilize plain paper using direct or indirect electrophotographic developing process, to developer which comprises the toner, and to an image-forming process and an image-forming apparatus using the developer which comprises the toner. It further relates to a toner for developing an electrostatic image used in full color copiers, full color laser printers and full color fax machines that utilize plain paper using the direct or indirect electrophotographic multicolor image-forming process, to a developer which comprises the toner, and to a image-forming process, a developing device (image-developer), an image-forming process and image-forming apparatus using the developer containing the toner.
2. Description of the Related Art
In a developing step, a developer used in electrophotography, electrostatic recording, electrostatic printing or the like, is first adhered to an image-bearing member such as a photoconductor on which a latent electrostatic image is formed. In a transferring step, the developer is then transferred from the photoconductor to a transferring medium such as a transfer paper, and is then fixed in an image-fixing step. In this procedure, the developer for developing an electrostatic image formed on the image-bearing surface of the transfer paper, may be a double-component developer comprising a carrier and a toner, or a single-component developer (magnetic toner/non-magnetic toner) which does not need a carrier.
Conventionally, dry toners used for electrophotography, electrostatic recording and electrostatic printing, are obtained by melt kneading a binder resin such as a styrene resin or a polyester resin with a coloring agent, and then pulverizing.
(Problems in Image-fixing)
After these dry toners are developed and transferred onto paper, or the like, the dry toners are fixed by heat fusion using a heating roller. If the temperature of the heating roller is too high, hot offset may occur in which excessive amount of toners become melted and stuck to the heating roller. Conversely, if the temperature of the heating roller is too low, the toners do not melt properly and thus image-fixing is poor. From the viewpoint of energy saving and size reduction of apparatus such as copiers or the like, a toner is desired to have higher offset temperature (heat-resistance offset property), and low image-fixing temperature (low temperature image-fixing properties). Moreover, storage heat resistance is required in which the toner does not block during storage and under the temperature conditions of the equipment used.
In full color copiers and full color printers, image glossiness and color mixing properties are required and the toner particularly needs to have a low melt viscosity. A sharp metal polyester binder resin has been therefore used. With such a toner, hot offset easily occurs, in the full color devices of the related art, the heating roller has been therefore coated with silicone oil. However, the process of applying the silicone oil to the heating roller requires an oil tank and oil coating equipment, which makes the apparatus complex and bigger. It has also led to deterioration of the heating roller and the need for maintenance has to be carried out periodically. Furthermore, adhesion of oil to copy papers or OHP (overhead projectors) film cannot be avoided. In OHP, particularly, there is a problem of poor color tone due to oil adhesion.
(Particle Diameter and Problems of Formation)
In order to obtain high image quality and high appearance quality, improvement has been made by making the particle diameter small, but with the usual manufacturing process of kneading and pulverizing, the particle formation is not defined. Inside the apparatus, the toner is stirred with the carrier in the developing part. In the case of a single-component developer, toner is further pulverized by contact stress with the development roller, the toner supplying roller, the layer thickness adjusting blade and the frictional charge blade. This produces submicron particles or results in having fluidizers embedded on the toner surface. An image quality therefore deteriorates. Also, due to the formation and poor fluidity (fluidability) of the toner as powder, the toner is required to be more fluidized, less of the toner is filled in the toner bottle, and it is therefore difficult to make the apparatus smaller.
In order to produce full color images, the transfer of multi-color toner from the photoconductor onto a transferring medium or paper is also complicated. Due to poor transfer properties resulting from the non-defined particle formation of the pulverized toner, there are problems that image dropout occurs, that more toner is required to cover the dropout, and the like.
Therefore, there has been an increasing demand on reducing the toner consumption by further improvement of transfer efficiency, to obtain high-quality images without image dropout, and to reduce running costs. If the transfer efficiency is very high, there is no need for a cleaning unit for removing non-transferred toner from the photoconductor or a transferring medium, and a smaller-sized apparatus can be attained as well as low cost. This also has the advantage that there would be no discarded toner. Thus, various processes have been developed to manufacture spherical toner, in order to compensate the disadvantages of toner having non-defined formation.
To achieve heat-resistant storage properties, low temperature image-fixing properties and hot offset-resistance properties, (1) a polyester resin partially crosslinked using a polyfunctional monomer Japanese Patent Application-Laid-Open GP-A) No 57-109825), (2) a urethane-modified polyester resin Japanese Patent Application Publication OP-B) No. 07-101318), and the like have been disclosed as binder resins. In addition, (3) toner obtained by granulating polyester resin fine particles and wax fine particles, has been disclosed to reduce the oil coating amount on heating rollers for full color image-forming (JP-A No. 07-56390).
To improve powder fluidity and transfer properties in the case of small particle diameter, there have been disclosed (4) a polymerized toner obtained by suspension polymerization of a vinyl monomer composition which contains a coloring agent, polar resin and release agent and is dispersed in water before the suspension polymerization (JP-A No. 09-43909), and (5) a toner comprised of a polyester resin having a spherical formation, using a solvent (JP-A No. 09-34167).
Furthermore, JP-A No. 11-133666, discloses (6) a substantially spherical toner that utilizes a polyester resin modified by urea bonds.
However, the toners disclosed in (1) to (3) all have poor powder fluidity and transfer properties, and decreasing the particle diameter does not allow high quality images. Further, regarding the toners of (1) and (2), heat storage properties and low temperature image-fixing properties cannot be obtained at the same time, and glossiness cannot be obtained with full color, so they were not practical. Regarding the toner of (3), low temperature image-fixing properties are inadequate and hot offset properties in oil-less image-fixing are unsatisfactory. The toners of (4) and (5) do have improved powder fluidity and transfer properties, however, for the toner of (4), low temperature image-fixing properties are poor and a large amount of energy was required for image-fixing. These problems are particularly evident for full color toners. For the toner of (5), low temperature image-fixing properties are much better, however, hot offset resistance is poor and when used for full color, oil coating of the heating roller cannot be dispensed with.
The toner of (6) has a viscoelasticity which can be suitably adjusted using a polyester resin extended by urea bonds, and it is thus excellent in the fact that suitable glossiness and mold release properties could both be realized, when used as a full color toner. In particular, an electrostatic offset, in which the image-fixing roller is charged, toners on a non-fixed image are electrostatically distributed, and toners adhere to the fixing roller, can be mitigated by the positive charge of the urea bond component and the weak negative charge of the polyester resin. However, despite these advantages, when the toners are actually used, the toners become more finely pulverized by mixing with the carriers in the developing part of the apparatus, and when used as a single-component developer, by contact stress due to the development roller, the toner supplying roller, the layer thickness adjusting blade and frictional charge blade, and produces particles. As the fluidizer becomes embedded in the toner surface, image quality tends to deteriorate, and the life of toner is thereby shortened.
(Problems of Image-Forming Process)
The above image deterioration with time is particularly remarkable when an image-forming process is used to increase magnetic brush density so as to prevent abnormal images such as “image omission at rear end.”
In general, in image-forming apparatuses for electrophotography or image-forming apparatuses for electrostatics such as copiers, printers and facsimile, and electrostatic recording image-forming apparatus, a latent electrostatic image corresponding to image information is first formed on a latent image-bearing member such as a photoconducting drum, photoconducting belt, or the like and then developed by a developing device to obtain a visible image. During this developing process, from the viewpoint of stability of development properties regarding transfer, half-tone reproducibility and temperature/humidity, an image-forming process employing a magnetic brush using a double-component developer which comprises a toner and a carrier, is generally utilized. In the other words, in this developing device, the double-component developer forms a brush chain on the developer bearing member, and in the developing region, toners in the developer are supplied to the latent image part on the latent image-bearing member. Here, developing region refers to a region where the magnetic brush are formed on the developer bearing member, and comes in contact with the latent image-bearing member.
The developer bearing member usually comprises a sleeve (development sleeve) formed in a cylindrical shape, and a magnet (magnetic roller) which generates a magnetic field to form the magnetic brush on the sleeve surface, is fitted inside the sleeve. In this process, the carriers form a magnetic brush on the sleeve along the magnetic force lines produced by the magnetic roller, and charged toners adheres to the carrier in the magnetic brushes. The magnetic roller comprises plural poles, and the magnets that generate these poles are arranged like rods. In particular, in the developing region on the sleeve surface, there is a developing main magnetic pole which forms the magnetic brushes. The developer forming the magnetic brushes on the sleeve surface can be moved by moving at least one of the sleeve and the magnetic roller. A developer transported to the developing region stands upwards so as to form magnetic brushes, along with the line of magnetic force generated by the developing main magnetic pole, the developer provided along with the line of magnetic force like a chain, comes in contact with the latent image-bearing member surface, so that it bends, and toners are supplied while brushing the latent electrostatic image based on the relative linear velocity difference between the developer brush in contact and the latent image-bearing member.
Conventionally, in this double-component developing process, developing conditions which allows sufficient image density are not compatible with those which allow low contrast images. It has been hence difficult to simultaneously improve high density parts and low density parts. The developing conditions which increase image density include (i) narrowing of the developing gap, which is the distance between the latent image-bearing member and a development sleeve, and (ii) widening of the developing region. On the other hand, the developing conditions which allow a low contrast image include (i) widening of the developing gap, and (ii′) narrowing of the developing region. In other words, these two sets of developing conditions are contradictory from each other, and are not compatible. Therefore, it is generally considered difficult to obtain a high quality image satisfying both sets of developing conditions over the whole range of the density. For example, if it is desired to emphasize low contrast images, an “image omission at rear end” where some image is missing from the back of a solid fill line cross part, black solid fill or half-tone solid fill image, often occurs. FIG. 1A shows an example of a fine solid image, and FIG. 1B shows an example of image omission at rear end. Also, some horizontal lines are thinner than vertical lines in a grid image formed with the same width, and small point images of one dot are not developed.
It is considered that this “image omission at rear end” occurs by the following mechanism.
First, referring into FIG. 2, the mechanism of an image-forming process using magnetic brushes formed of a double-component developer, will be described. FIG. 2 shows an example of a negative-positive developing region, which shows an example of the above-mentioned image-forming process. In FIG. 2, the development roller which serves as a developer-bearing member is shown on the right-hand side, and the photoconductor P which serves as the latent electrostatic image-bearing member is shown on the left-hand side. The development roller comprises a development sleeve which moves in a direction D, and a development magnet fixed therein. Due to the movement of the development sleeve, the double-component developer comprising a non-magnetic toner and a magnetic carrier, is transported in a vicinity of a part adjacent to the photoconductor. When the double-component developer reaches the vicinity of the part adjacent to the photoconductor P, the carrier stands upwards and forms a magnetic brush due to the magnetic force of the magnetic pole for development. In FIG. 2, small dots express toners, and large dots express carriers. For simplicity, only one magnetic brush is shown by solid lines in the part adjacent to the photoconductor P. Herein, the remaining magnetic brushes are shown by dotted lines and the toners are omitted from the figure.
At the same time, the photoconductor rotates in the direction C while having the latent electrostatic image on a surface thereof. In FIG. 2, in the latent electrostatic image, a non-imaging part is charged negatively as shown by “A.” At the part where the photoconductor faces the development roller, the magnetic brushes are contacted onto a latent image on the photoconductor, and the toners are disposed on the latent image by development electric field. As a result, a toner image is formed in the developing part of the latent image on the photoconductor downstream of the developing part as shown by B. Hereinafter, the length over which the magnetic brush contacts the photoconductor along a surface of the photoconductor in the direction that the photoconductor moves will be referred to as the development nip. It should be noted that, if only one point of the developer-bearing member contacts one point of the photoconductor, a sufficient image density cannot be obtained, hence a speed difference is generally allowed between the photoconductor and development sleeve so that a certain area of the developer-bearing member contacts one point on the photoconductor. The development sleeve therefore moves earlier than the photoconductor.
The mechanism whereby the image omission at rear end shown in FIG. 3 will now be described, referring the image-forming process using the double-component developer shown in FIG. 2 as to an example. FIGS. 3A through 3C each show examples of enlargements of the part adjacent to the photoconductor and the development sleeve in FIG. 2. In the FIGS. 3A through 3C, the tip of the magnetic brush shown on the right-hand side of the figures approaches the photoconductor shown on the left-hand side. FIGS. 3A through 3C each show the movement of the magnetic brush in time series, starting from FIG. 3A. In FIGS. 3A through 3C, the part adjacent to the photoconductor and the development roller is in the step of developing the boundary between the non-imaging part and a black solid image, i.e., the state in which the “image omission at rear end” appears, and the toner image which has just been developed is formed downstream of a direction that the photoconductor rotates. One of the magnetic brushes on the development sleeve is approaching the photoconductor in this state. Here, the photoconductor rotates clockwise, and as the development sleeve moves earlier than the photoconductor as described the above, the magnetic brush catches up with and passes the photoconductor. Therefore, in FIGS. 3A through 3C, the photoconductor is depicted as stationary to simplify the model. In FIG. 3A, the magnetic brush which approaches the photoconductor passes through a non-imaging part up to a point E, which is to be developed, and due to a repulsion F between negative charges, toners gradually leave the photoconductor and moves towards the development sleeve.
This phenomenon is referred to hereafter as “toner drift.” As a result of the toner drift, when the magnetic brush reaches the point E, the magnetic brush adjacent to the photoconductor have the positively-charged carriers directly present as shown in FIG. 3B. As a result, there is no toner disposing on the latent image at the point E, and the point E is not developed. Also, when the magnetic brush reaches the point G in FIG. 3C, if the disposing force between the toner and photoconductor is weak, toner which once disposed on the photoconductor may be disposed again to the carrier due to electrostatic force. As a result, at the boundary between the image part and non-imaging part, developing does not take place and this causes the “image omission at rear end.”
The mechanism of image omission at rear end has been described referring to one cross-section of the part adjacent to the development roller and photoconductor. However, in practice, when the magnetic brushes contacts the photoconductor in the longitudinal direction of the development roller, the length of the magnetic brush is not the same among the magnetic brushes, and magnetic brushes have different size, depending on the position in the longitudinal direction of the development roller. FIG. 4 shows this situation. FIGS. 4A and 4B each schematically shows an example of the state of the magnetic brush when the photoconductor is not present. FIG. 4A shows a magnetic brushes present on the development roller in the longitudinal direction. FIG. 4B shows an example of a cross-section of the magnetic brush in FIG. 4A taken along a plane H-H′ perpendicular to the longitudinal direction. In other words, FIG. 4B is a view which shows the magnetic brush in the same cross-section as that of FIG. 2. In order to clarify the relation with other drawings, FIG. 4A schematically shows the positional relationship with the photoconductor. As shown in FIG. 4A, there is a large distribution in height of the magnetic brushes present in the longitudinal direction. This means that the magnetic brushes contact the latent image-bearing member irregularly in the longitudinal direction. As a result, there is also distribution as regards the degree of toner drift in the longitudinal direction and the degree of “image omission at rear end” in the longitudinal direction are not fixed either, hence, a zigzag image omission at rear end appears in the longitudinal direction of the development roller, as shown in FIGS. 1A and 1B.
Due to a similar mechanism, horizontal lines are thinner than vertical lines (horizontal line thinning) and the formation of isolated dots is unstable, which makes it difficult to obtain high image quality by the development using magnetic brushes formed of a double-component developer.
An effective way of preventing abnormal images such as “image omission at rear end,” and obtaining a high-quality image with good horizontal line and dot reproducibility without edge influence, is to arrange the developing device so that, in the development nip region where the magnetic brush on the development sleeve contacts the photoconductor during developing, the development nip region is narrowed. The principle of this is that, if the nip in the developing part is made narrower, the time for which the magnetic brush contacts the non-imaging part is short, which is considered to reduce the toner drift.
FIGS. 5A through 5C each show the above situation. FIGS. 5A through 5C are each a view showing an example of development when the nip in FIGS. 3A through 3C is narrowed. Specifically, in FIG. 5, unlike the case of FIGS. 3A through 3C, the magnetic brush contacts the photoconductor in a shorter time so that toner drift is reduced, in FIG. 5B, as toner drift is reduced, toners are applied to the position E, and in FIG. 5C, toners on the photoconductor are not disposed again on the carriers, because the carriers are not directly present. For this reason, image omission at rear end can be reduced. To narrow the nip, it is effective to decrease the half-value width of the magnetic pole for development. Herein, the half-value width is a value of the angular width of a part showing half of the maximum normal magnetic force (peak) of the magnetic force distribution curve in the normal direction of the magnetic pole for development. For example, if the maximum normal magnetic force of a magnet formed by the N pole is 120 MT, this is an angular width of a part showing a value of 60 mT.
However, it is known that the image omission at rear end cannot be completely suppressed merely by decreasing the half-value width of the magnetic pole for development. It is assumingly because that the nip cannot be narrowed at all positions in the longitudinal direction. Specifically, as shown in FIGS. 4A and 4B, there is usually some distribution in the height of the magnetic brushes present in the longitudinal direction, and if there is a part where long magnetic brushes are present in the longitudinal direction, the nip cannot be narrowed in this part, so toner drift cannot be avoided. To deal with this problem, it has been disclosed and applied to suitably position the magnet forming the magnetic pole in the development sleeve so that the magnetic flux density in the development nip is in the dense direction, or the attenuation factor of magnetic flux density in the normal direction in the developing main magnetic pole is above a specific value, and image omission at rear end is not severe (refer to, for example, Japanese Patent Application Laid-Open UP-A) No. 2000-305360).
In such a developing device (image-developer), in the nip region where the magnetic brush contacts the latent image-bearing member, the magnetic brush is formed with a uniform density in the longitudinal direction, so distribution in the height of the magnetic brushes in the longitudinal direction can be prevented.
The prevention of distribution in the height of the magnetic brushes in the longitudinal direction by densely forming the magnetic brush, is shown in FIGS. 6A and 6B. FIG. 6A shows an example of magnetic brushes formed densely, and FIG. 6B shows an example of magnetic brushes formed with the distribution of height. In FIG. 6A, the magnetic brushes are formed densely, so the distribution in the height of the magnetic brush in the longitudinal direction is decreased, and as a result, an image without “image omission at rear end” can be obtained as shown in FIG. 6A. On the other hand, FIG. 6B shows an example of the magnetic brushes in the related art that have distribution in the height. If the magnetic brushes as shown in FIG. 6B are used, “image omission at rear end” occurs as shown therein. Hence, if the magnetic brushes are formed with sufficient density upon reaching the nip, distribution in the height of the magnetic brush in the longitudinal direction is reduced, and as the magnetic brushes enter the nip in a sufficiently uniform state in the longitudinal direction, toner drift at various positions in the longitudinal direction can be reduced, and the occurrence of “image omission at rear end” at various positions in the longitudinal direction is sufficiently reduced.
Herein, to form the magnetic brush densely, the attenuation factor of the normal magnetic flux density of the magnetic pole for development forming the magnetic brush may be increased. The attenuation factor of the normal magnetic flux density of the magnetic pole for development is a value obtained by: (x−y)+x×100%, which expresses how much the normal magnetic flux density “y” is attenuated in a 1 mm distant part from a surface of the development roller relative to the normal magnetic flux density “x” of the surface of the development roller. For example, when the normal magnetic flux density of the surface of the development roller is 100 mT and the normal magnetic flux density in a 1 mm distant part from the surface of the development roller is 80 mT, the attenuation factor is 20%. The normal magnetic flux density is measured by for example a Gauss meter (HGM-8300: produced by ADS (Application & Data System, Inc.)) and an A1 axial probe (produced by ADS (Application & Data System, Inc.)). It has previously been disclosed that if the attenuation factor of the normal magnetic flux density of the main magnetic pole which generates the brush in the developing region is 40% or more, and preferably 50% or more, a magnetic brush having more density is formed, and the more the distribution in the height of the magnetic brushes in the longitudinal direction can be reduced (refer to, for example, JP-A No 2000-305360). According to the present invention, as an attenuation factor within this range is effective, a developing device which realizes this attenuation factor is used.
The reason why the magnetic brushes become denser when the attenuation factor increases, is considered to be that when the attenuation factor is high, the magnetic force sharply decreases with increasing distance from the development roller, so the magnetic force at the tip of magnetic brushes becomes too weak to maintain the magnetic brush, and carrier at the magnetic brush tip is attracted to the surface of the development roller where the magnetic force is strong. The attenuation factor can be increased by selecting the material for magnet which forms a magnetic pole for development, or by concentrating the magnetic force lines leaving the magnetic pole for development. Of these methods, the magnetic force lines leaving the magnetic pole for development can be concentrated for example by forming the magnetic pole for development from a main magnetic pole which forms the magnetic brushes, and auxiliary magnetic poles having opposite polarity to the main magnetic pole disposed upstream and downstream of the main magnetic pole in the direction that the developer-bearing member moves.
Another solution of concentrating the magnetic force lines leaving the magnetic pole for development, when there is an additional magnetic pole to the magnetic pole for development in the developer-bearing member, such as a transport magnetic pole, is to concentrate the majority of the magnetic force lines leaving the magnetic pole for development in the transport magnetic pole by narrowing the half-value width of the magnetic pole for development. It is preferable that this half-value width is 22° or less, and preferably 18° or less. It has been experimentally verified that this attenuation factor increases when the half-value width of the magnetic pole is narrowed.
Summarizing the above, by using a double-component magnetic brush developing device (image-developer) which has functions of: (1) magnetic brushes are formed uniformly in the longitudinal direction to come in contact with a photoconductor; (2) an auxiliary magnetic pole is formed which assists the magnetic force of the main magnetic pole for development; (3) the attenuation factor of the normal magnetic flux density of the main magnetic pole is 40% or more; and (4) the half-value width of the main magnetic pole is 22° or less, abnormal images having “image omission at rear end” can be prevented, and high image quality with sufficient image density can be achieved.
However, if the above image-forming process (1), (2), (3), and (4) which increase the magnetic brush density to prevent abnormal images that have “image omission at rear end” is employed, the developer in the development nip part has a higher contacting force (impact force) given on the photoconductor, compared to the case when the magnetic brush density is low, and a high stress is easily given on the developer (and toners contained in the developer), so the toners tend to deteriorate with time, charge is lost and toner scattering or toner deposition on background of the image tend to occur. Due to this, image deterioration with time as compared to the initial image, becomes much more apparent. In particular, when a toner having a relatively wide toner charge distribution is used, this is a very serious problem. Accordingly, when an image-forming process which increases magnetic brush density is adopted to prevent abnormal images having “image omission at rear end,” it is important to prevent image deterioration with time.